Engines may be configured with exhaust gas recirculation (EGR) systems to recirculate a portion of exhaust gas from an engine exhaust to an engine intake system. By providing a desired engine dilution, such systems reduce combustion temperatures and throttling losses. Consequently fuel economy and vehicle emissions are improved. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the engine intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions.
Various approaches may be used to turn off EGR in such dedicated EGR systems during conditions when EGR is not required such as during transients such as start-stop and deceleration fuel shut off (DFSO) operations, for example. One example approach includes the use of diverter valves for diverting some or all of the exhaust from the dedicated EGR cylinder to an exhaust location. Another example approach, shown by Chapel et al. in WO 2013175091, inhibits combustion in the dedicated EGR cylinder by blocking fuel injection when an engine temperature is lower than a threshold temperature.
However, the inventors herein have recognized potential issues with the above approaches. As an example, valve actuators and fuel injection actuators require a duration of time to achieve a desired position. Consequently, EGR may not be turned off immediately. Further, even after the actuators have reached the desired position, there may be a delay in purging EGR from the engine intake. If the engine is shut down before EGR is purged from the intake manifold, there may be excess dilution of intake air with EGR during engine restart. The presence of excess intake air-EGR dilution may increase combustion instability issues and the propensity for engine misfires during engine restart.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: deactivating a dedicated EGR cylinder group of a multi-cylinder engine in response to an imminent engine shutdown condition; monitoring an EGR amount after deactivating; and shutting down the engine in response to the monitored EGR amount falling below a threshold.
In this way, EGR may be purged from the intake before shutdown. The shutting down of the engine may include deactivating spark and/or fuel injection to all cylinders, for example.
As an example, an engine system may be configured with a single dedicated EGR (DEGR) cylinder for providing external EGR to all engine cylinders. During selected conditions when an engine shutdown is requested, detected, or anticipated, fueling to the DEGR cylinder may be stopped while operating the remaining non-DEGR cylinders with fueling. After EGR is purged from the intake manifold and/or the EGR system, intake and/or exhaust valves at the DEGR cylinder may be deactivated. Further, the non-DEGR cylinders may be deactivated (by stopping fueling and/or valve deactivation, for example) and the engine may be commanded to shut down and spin down to rest. Subsequently, during an engine restart operation from rest, the DEGR cylinder may be operated to combust only after the engine has completed cranking and a stable engine speed is established.
In one example, when a DFSO condition is detected, fueling to the DEGR cylinder may be stopped prior to stopping fueling to the non-DEGR cylinders (e.g., prior to entering DFSO). An intake EGR rate may be monitored upon stopping fueling to the DEGR cylinders. When the intake EGR rate decreases below a threshold rate, fueling to the non-DEGR cylinders may be stopped.
In this way, EGR may be purged from the intake manifold prior and/or the EGR system to shutting down the engine or prior to entering DFSO. By purging EGR, excess dilution of the intake air during subsequent engine restart or during resuming engine operation from DFSO may be reduced. As a result, robust and repeatable engine starts on an engine system with dedicated EGR may be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.